The present invention is directed to internally enhanced heat transfer tubes, and more particularly, to an arrangement of roughness elements on the internal surface of the heat transfer tube which provides more efficient and economical heat transfer.
It is highly desirable to limit the material content of the heat transfer tube, particularly as the material in the roughness elements increases the cost of the heat transfer tube. On the other hand, the size, shape and spacing of the roughness elements can be optimized to maximize heat transfer efficiency for all types of tubing used in refrigeration systems. The enhancements, such as roughness elements, on the internal surface of a heat transfer tube are typically formed by deformation of material. Previous internal enhancement arrangements have not optimally maximized heat transfer efficiency while minimizing material content.
For example, U.S. Pat. Nos. 4,794,983 and 4,880,054 show projected parts having cavities on the inner wall surface of a tubular body. The ratio of the interval (P) between the projected parts and the height (e) of the projected parts must satisfy the equation 10.ltoreq.P/H.ltoreq.20.
U.S. Pat. No. 4,402,359 shows pyramid fins formed integrally on the outer surface of a cylindrical tube. The preferred height of the pyramid fins is about 0.022 inches at 20 threads per inch.
U.S. Pat. No. 3,684,007 shows a smooth, flat surface having a multiplicity of discrete raised sections in the general shape of pyramids.
U.S. Pat. No. 4,216,826 is an example of an external tube surface including thin walled fins of rectangular cross-section which are about 0.1 millimeters thick and about 0.25 millimeters high.
U.S. Pat. No. 4,245,695 shows the external surface of a heat transfer tube including pyramid like raised sections with a cylindrical shape. In an experimental example this patent describes a "circular pitch" of 1.41 millimeter and a 0.75 millimeter height for the raised parts.
U.S. Pat. No. 4,733,698 shows a complex internal groove arrangement which includes projecting portions having a triangular cross-section.
U.S. Pat. No. 4,715,436 shows a row of projections regularly spaced on the inner surface of a heat transfer tube. Each projection is composed of a smooth curved surface formed by external deformation of the tube walls. The smallest pitch to height ratio shown is 5.6 (Z/E=2.45/0.45).
U.S. Pat. No. 4,330,036 is similar to the '436 patent in showing a number of beads on the internal surface of a heat transfer pipe.
U.S. Pat. Nos. 4,660,630 and 4,658,892 are examples of internally finned tubes showing spiral grooves separated by continuous ridges.